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Skin Cancer Module: Practice Exercises

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Parts of a Cell
How Cells Divide

Module 2: Normal Cell Biology

Objective: Learn about the biology of normal cells to be able to contrast them to cancer cells.

The Parts of a Cell

Cells have two main parts, the cytoplasm and the nucleus. The cytoplasm surrounds the nucleus, much the way the white surrounds the yolk in an egg. The nucleus is separated from the cytoplasm by the nuclear membrane. The cell membrane surrounds the cytoplasm.

The Cytoplasm

The cytoplasm contains many organelles. Organelles are like tiny organs. Each has specific jobs to do within the cell. Some of the important organelles are described here.

  • The endoplasmic reticulum helps make protein.
  • The golgi apparatus helps move materials out of the cells in which they are made.
  • Mitochondria make energy needed for cell function.
  • Lysosomes digest substances brought into the cell.

The substance surrounding organelles within the cytoplasm is known as the cytosol.

The Nucleus

The nucleus plays an important role in heredity and cell division.

  • Heredity is what you "inherit" from your parents through your genes. (Genes are found in chromosomes.)
  • Cell division is how new cells are made.

The nucleus contains deoxyribonucleic acid, or DNA. Together with proteins called histones, DNA makes up the cell's chromosomes

Human cells normally contain 46 chromosomes arranged into 23 pairs. In 22 of the pairs the partner chromosomes are similar. These chromosomes are called autosomes. The 23 twenty-third pair makes up the sex chromosomes, called "X" and "Y." Females have two X chromosomes. Males have an X and a Y chromosome.

Two Types of Nuclei/Two Types of Cells

  • Cells that make up the body contain the usual 46 chromosomes. They are called somatic cells.
  • Sex cells (the egg and sperm) contain only 23 chromosomes. That's half the number found in somatic cells. These cells are called gametes.

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How Cells Divide

There are two types of cell division, mitosis and meiosis.

  • Mitosis is how somatic cells divide.
    During mitosis, the DNA doubles. This doubles the number of chromosomes. The nucleus then splits in two, followed by the cytoplasm. The cell membrane closes around each of the two new cells, separating them. After mitotic cell division each "daughter" cell has the correct number of 46 chromosomes.

  • Meiosis is how sex cells divide.
    In meiosis, the chromosomes do not double before the nucleus splits. After the cell divides, each resulting egg or sperm cell has only 23 chromosomes. This is half the number of chromosomes in a normal cell.

During fertilization, a woman's egg and a man's sperm join. The resulting cell has the correct number of 46 chromosomes. This new cell is called a zygote.

What Makes Cells Divide?

Growth factors in the blood or produced by cells stimulate cells to divide. Certain genes in the cell then turn the cell "on" so that division can happen. After the cell has divided, other genes turn the cell "off" again.

The chain of events is as follows:

  1. Growth factors attach to the cell membrane. They turn "on" messenger substances within the cell.
  2. The messengers send signals to the nucleus of the cell.
  3. Genes in the nucleus turn "on" the division process.
  4. The DNA in the nucleus replicates (doubles).
  5. The cell divides.
  6. Genes in the nucleus turn the cell "off."

Changes (mutations) in the genes can affect their ability to turn the cells "on" or "off." This can cause uncontrolled cell growth and cancer.


Step 1: Go to* to learn more about cell biology.
Step 2: Take the quiz to see how much you have learned.

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* Links to non-Federal organizations are provided solely as a service to our users. Links do not constitute an endorsement of any organization by CDC or the Federal Government, and none should be inferred. The CDC is not responsible for the content of the individual organization Web pages found at these links.

This page last reviewed April 24, 2007

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